Discontinue HHVM nightly support on Precise

See travis-ci/travis-ci#3788
and facebook/hhvm#5220

lib/travis/build/script/php.rb

@@ -81,6 +81,10 @@ def configure_hhvm
         end
 
         def install_hhvm_nightly
+          sh.if '$(lsb_release -sc) = "precise"' do
+            sh.echo "HHVM nightly is no longer supported on Ubuntu Precise. See https://github.com/travis-ci/travis-ci/issues/3788 and https://github.com/facebook/hhvm/issues/5220", ansi: :yellow
+            sh.raw "travis_terminate 0"
+          end
           sh.echo 'Installing HHVM nightly', ansi: :yellow
           sh.cmd 'sudo apt-get update -qq'
           sh.cmd 'sudo apt-get install hhvm-nightly 2>&1 >/dev/null'