PHPLIB-519: Allow testing against development driver versions

[alcaeus]

This allows us to later switch to always test against dev driver versions until the RC phase of the extension, saving us the trouble of releasing alpha builds of the driver for internal testing purposes.

[jmikola]

I actually don't recall why we stopped using the tpecl command, which caches the install. It looks like it was removed in 8774590.

.travis.yml still has the tpecl definition, but it looks like it's no longer being used since that commit. Do you want to try relocating it to install-extension.sh and use it when installing based on $DRIVER_VERSION? Otherwise, I suppose we should just delete it entirely (can do so in a separate issue/PR).

[alcaeus]

I'll change it back to using tpecl, makes sense to cache it when possible.

[jmikola]

As a sanity check, I think it would make sense to raise an error at the top of this script if both $DRIVER_VERSION and $DRIVER_BRANCH are non-empty. As-is, we're likely to compile the driver twice and end up with the branch version (last one wins).

[alcaeus]

I'll change this around to use an else if and give precedence to a branch build. This allows us to omit setting DRIVER_VERSION="" in the env for branch builds. If none of the drivers can be installed, the following php --ri call will fail, but I can also exit with a non-zero code in this script to break the build earlier.