[ci] Fix ipopt build for OSX

See: robotology/homebrew-formulae#12

Author: bchretien

.travis.yml

@@ -3,7 +3,7 @@ language: cpp
 env:
   global:
   - APT_DEPENDENCIES="cmake cmake-data doxygen libltdl-dev libboost-all-dev liblog4cxx10-dev"
-  - HOMEBREW_DEPENDENCIES="doxygen log4cxx ipopt openblas mumps"
+  - HOMEBREW_DEPENDENCIES="doxygen log4cxx dartsim/dart/ipopt openblas mumps"
   - GIT_DEPENDENCIES="roboptim/roboptim-core"
   - DEBSIGN_KEYID=5AE5CD75
   - PPA_URI="roboptim/ppa"