Solution: establish a matrix of CI options. On one axis we have the
build system (autotools, cmake, android) and on the other axis we
have the encryption options (tweetnacl, libsodium or none).
Solution: checkout stable branch instead.
Several warnings are printed when building from the master branch,
and developers recommend using the stable branch instead.
Currently, a test fails on Travis-CI but that can not be reproduced locally.
Without the contents of the test-suite.log, this is difficult to analyze.
Solution: print test-suite.log if "make check" fails
This is based on the following answer on SO:
http://stackoverflow.com/a/32597140
