Revert "Remove gswe_init() call from GsweTimestamp initializers"

This reverts commit 078ff3370d6820800accb8c2f9c69285796cf99b.

It turns out that we actually need to call gswe_init() (or at least
g_type_init()) for GObjects to work.
Fix comment in gswe_init_with_dir()
Remove make check from Travis config

The checks are not working yet, thus Travis is continuously fails.