Remove or fix binary builds

[MartijnR]

These were added to be used in pyxform (initially as requested and funded by OpenClinica). However, it really makes more sense to run pyxform and a validator separately and that's what OpenClinica is doing.

I think nobody uses enketo-validate inside pyxform.

• create issue in xlsform/pyxform to propose removing the enketo-validate integration
• remove the code in .travis.yml and the build-binary.sh file