Force Ubuntu "precise" for Travis CI builds.

We've been seeing strange errors in jemalloc_cpp.cpp since Travis upgraded from
precise to trusty as their default CI environment (seeming to stem from some
the new clang version finding the headers for an old version of libstdc++.  In
the long run we'll have to deal with this "for real", but at that point we may
have a better C++ story in general, making it a moot point.

.travis.yml

@@ -1,4 +1,5 @@
 language: generic
+dist: precise
 
 matrix:
   include: