[PHP] re-order dependency installations in order to optimise building caches

[itscaro]

Description

When dependencies/requirements are not changed, these build steps should be cached.

Motivation and Context

• [x] I have raised an issue to propose this change (https://github.com/openfaas/templates/issues/97)

Which issue(s) this PR fixes

Close #97

How Has This Been Tested?

Manually by changing Dockerfile

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.