Upgrade to Elixir 1.7

[indocomsoft]

[jiachen247]

note that one dependency fails to compile once upgraded to Elixir 1.7

meh/exquisite#10

[jiachen247]

getsentry/sentry-elixir#309

New version of sentry which does not log NoRouteFound errors depends on elixir 1.7

To plan for upgrade asap.

[indocomsoft]

Dependent on sheharyarn/memento#2

[jiachen247]

Resolved with #343