Travis: Add /opt/local/bin to PATH for Mac build

(oversight from previous commit)

macports-ci does this, and it's necessary in order for the build script
to find md5sum.
diff --git a/.travis.yml b/.travis.yml
index 6eb23b3..a5147b0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -68,6 +68,7 @@
       sudo installer -pkg MacPorts-2.6.4-10.12-Sierra.pkg -target / &&
       rm MacPorts-2.6.4-10.12-Sierra.pkg &&
       sudo /opt/local/bin/port -N install gcc5 yasm md5sha1sum &&
+      export PATH=/opt/local/bin:$PATH &&
       popd &&
       git clone --depth=1 https://github.com/libjpeg-turbo/gas-preprocessor.git ~/src/gas-preprocessor &&
       ln -fs /Applications/Xcode.app /Applications/Xcode72.app;