configure: Accept LLVM 3.4.X during configuration

The previous regex was a bit to strict, rejecting versions such as 3.4.1 which
is apparently the version which travis is currently installing, causing all
travis builds to fail.
auto merge of #13216 : alexcrichton/rust/fix-configure-for-travis, r=sfackler

The previous regex was a bit to strict, rejecting versions such as 3.4.1 which
is apparently the version which travis is currently installing, causing all
travis builds to fail.