Transifex: Don't push translation

A Travis CI bot automatically extracts the source strings and pushes
them to Transifex. Previously, this bot also pushed translations, which
does not make sense, as the translations are made at Transifex.
Surprisingly, this didn't seem to cause much trouble - my best guess is
that Transifex respects modification times and ignored the pushed
translations. It certainly does so for source files.
But @mvantellingen reported that Dutch source files were overwritten. I
just did a manual push of only source files, and the Transifex UI still
shows all languages as updated. It makes sense as the source strings
changed, but makes me wonder if Michael saw actual overwrites.

Either way, pushing translations was the wrong thing to do. And as
nothing should go wrong with the English source file, I removed the skip
flag.

I intent to re-enable the bot once we've released 0.8.