move rebar config to script
move meck to test dependency
  * mix dep only in test env
  * rebar.config.script checks REBAR_ENV env var
  * update travis config
  * make elixir version configurable with travix mix builds
    and rebar script
  * add elixir versions to test configs
improve compatibility
  % for regex sigil and // for defaults are deprecated
  in Elixir 0.12.5, but the replacements (~ and \\) aren't
  supported in 0.12.0.  Solution? Remove use of these features.