implement older compilers (clang 3.[0..5] and gcc 4.7)

[odinthenerd]

looking at the libs on here there seems to be a lot of support for compilers which are older than those currently offered. I am looking to add a legacy mode to boost.tmp (same front end different back end) using this pattern https://godbolt.org/g/W62iut and I would like to bench it against others on here (initially its probably going to suck). would it be feasible to add a few more clang versions and g++-4.7 ?

mp11:
g++ 4.7 or later
clang++ 3.3 or later
Visual Studio 2013, 2015, 2017

metal:
GCC | 4.7 | Ubuntu 14.04 LTS
Clang | 3.4 | Ubuntu 14.04 LTS
Clang | 3.5 | Ubuntu 14.04 LTS

meta:
clang >= 3.4

brigand:
clang >= 3.4

[ldionne]

Feel free to submit a PR that adds new compilers to the Travis. Just make sure libraries that don't support those compilers are not tested on them (or they'll fail).

I think it would also be good not to add too many legacy compilers, since this will increase the number of jobs. We should be slightly careful about this.

[odinthenerd]

I'll see if I can find time, I agree that we shouldn't murder Travis