Switch to C language in travis config

I've proposed this in the past (#689) but @nikic and @smalyshev was against it on the basis that installing the deps manually would be more fragile and could slow down the build.
I disagree that it would make our travis config more fragile as mentioned in the previous PR, having more explicit dependencies makes it much easier to reproduce the travis builds locally and also less moving parts for our tests (for example our test suite picks up ini settings from the system wide php installation, so that's another part where a change not controlled by us can change the result of our tests).
The missing deps which have to be additionally installed are 5 packages, and the installation takes about 20 seconds, which compared to our average build times (~30 minutes per configuration) is negligible imo.
There is another reason for me to propose this change again: travis introduced a beta multi-os support which adds the capability of building projects not just on linux but also on Mac.
Unfortunately php projects (travis projects using language: php in their travis config) doesn't supported yet, but we could opt-in to this feature if we turn our project to a C one (which would be the appropriate settings anyways).