Automate releases (part 1)

[misaochan]

Part of the changes needed for #1056 . I have encrypted the keystore, modified .travis.yml to handle that via Travis cmd line, and added the env vars.

[codecov-io]

Codecov Report

Merging #2111 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #2111      +/-   ##
=========================================
+ Coverage     5.6%   5.68%   +0.07%     
=========================================
  Files         225     225              
  Lines       11379   11418      +39     
  Branches     1065    1070       +5     
=========================================
+ Hits          638     649      +11     
- Misses      10689   10715      +26     
- Partials       52      54       +2

Impacted Files	Coverage Δ
...n/java/fr/free/nrw/commons/CommonsApplication.java	42.85% <0%> (-1.73%)	⬇️
...explore/recentsearches/RecentSearchesFragment.java	0% <0%> (ø)	⬆️

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