Add Travis integration

Add Travis integration

Based on patch from @peteut

Squashed commit of the following:

commit ee62c49c90d58a7c9b5cc6b410e945c3cb0caabf
Author: Nick Gasson <nick@nickg.me.uk>
Date:   Sun Nov 6 14:29:10 2016 +0000

    Add badge and VHPI

commit a7999314a9c2f46b73db88b39b095cae71b051c0
Author: Nick Gasson <nick@nickg.me.uk>
Date:   Sun Nov 6 14:25:43 2016 +0000

    Fix some warnings in run_regr.c

commit fb7a5df2e48689edbcc138a3380e5cf11273b4b2
Author: Nick Gasson <nick@nickg.me.uk>
Date:   Sun Nov 6 14:20:02 2016 +0000

    Add configure check for pow() function

commit 748fa2fa7feb2e47a664f6f62e58ab89fa2a364f
Author: Nick Gasson <nick@nickg.me.uk>
Date:   Sun Nov 6 14:11:14 2016 +0000

    Use LLVM 3.4

commit bd7939413ed6825092a3289457264b263b7a4c8e
Author: Nick Gasson <nick@nickg.me.uk>
Date:   Sun Nov 6 14:07:05 2016 +0000

    Add Travis config file