tools/anitya-watch: ignore pre-release versions

i have no some idea how lua works, here's the messages anitya sends via zmq:

new package

{
  distro = {
    name = "Alpine",
    <metatable> = <1>{
      __name = "object"
    }
  },
  message = {
    agent = "ptrcnull",
    distro = "Alpine",
    new = "sbctl",
    project = "sbctl",
    <metatable> = <table 1>
  },
  project = {
    backend = "GitHub",
    created_on = 1647305303,
    ecosystem = "https://github.com/Foxboron/sbctl",
    homepage = "https://github.com/Foxboron/sbctl",
    id = 242865,
    name = "sbctl",
    regex = <2>{ -- null
      <metatable> = {
        __name = "null",
        __tostring = <function 1>
      }
    },
    stable_versions = {
      <metatable> = <3>{
        __name = "array"
      }
    },
    updated_on = 1647305303,
    version = <table 2>,
    version_url = <table 2>,
    versions = {
      <metatable> = <table 3>
    },
    <metatable> = <table 1>
  },
  <metatable> = <table 1>
}

version update

{
  distro = <1>{ -- null
    <metatable> = {
      __name = "null",
      __tostring = <function 1>
    }
  },
  message = {
    agent = "anitya",
    ecosystem = "https://github.com/Foxboron/sbctl",
    odd_change = false,
    old_version = "",
    packages = { {
        distro = "Alpine",
        package_name = "sbctl",
        <metatable> = <2>{
          __name = "object"
        }
      },
      <metatable> = <3>{
        __name = "array"
      }
    },
    project = {
      backend = "GitHub",
      created_on = 1647305303,
      ecosystem = "https://github.com/Foxboron/sbctl",
      homepage = "https://github.com/Foxboron/sbctl",
      id = 242865,
      name = "sbctl",
      regex = <table 1>,
      stable_versions = { "0.8", "0.7", "0.6", "0.5", "0.4", "0.3", "0.2", "0.1",
        <metatable> = <table 3>
      },
      updated_on = 1647305322,
      version = "0.8",
      version_url = <table 1>,
      versions = { "0.8", "0.7", "0.6", "0.5", "0.4", "0.3", "0.2", "0.1",
        <metatable> = <table 3>
      },
      <metatable> = <table 2>
    },
    stable_versions = { "0.8", "0.7", "0.6", "0.5", "0.4", "0.3", "0.2", "0.1",
      <metatable> = <table 3>
    },
    upstream_version = "0.8",
    versions = { "0.8", "0.7", "0.6", "0.5", "0.4", "0.3", "0.2", "0.1",
      <metatable> = <table 3>
    },
    <metatable> = <table 2>
  },
  project = {
    backend = "GitHub",
    created_on = 1647305303,
    ecosystem = "https://github.com/Foxboron/sbctl",
    homepage = "https://github.com/Foxboron/sbctl",
    id = 242865,
    name = "sbctl",
    regex = <table 1>,
    stable_versions = { "0.8", "0.7", "0.6", "0.5", "0.4", "0.3", "0.2", "0.1",
      <metatable> = <table 3>
    },
    updated_on = 1647305322,
    version = "0.8",
    version_url = <table 1>,
    versions = { "0.8", "0.7", "0.6", "0.5", "0.4", "0.3", "0.2", "0.1",
      <metatable> = <table 3>
    },
    <metatable> = <table 2>
  },
  <metatable> = <table 2>
}

closes #45 (closed)