Plugin & definition don't need to be executable

These shell scriptlets are only meant to be included by other shell
scripts to set up some variables and a few helper shell functions.
Merge branch 'fix-permissions' of https://github.com/gerardroche/php-build into gerardroche-fix-permissions
Fix permissions (-x) on some definition files.
The definition files do not need to be executable anymore.

Update the CONTRIBUTING.md file accordingly.