switch glibc build from ubuntu12.04 to centos6
switch glibc build from ubuntu12.04 to centos6

centos6 has older glibc, and not eglibc. We were previously blocked from
building by a mysterious build failure. Turns out the issue is that the
OPAM environment isn't active. Now we ensure the env is active, and the
build succeeds!

justjake/encabulator,justjake/encabulator