Update mix.exs

Updated dependencies for new version of httpoison
Merge pull request #7 from riverrun/master

Four types of requests added
Merge pull request #8 from riverrun/master

Added query constraints (filters)
check if system variable unset
Merge branch 'raise-warnings'
add ParseClient doctests
Merge pull request #9 from mutablestate/master

Raise error if required system variables unset
Added tests for parsing of filters and options