Travis build failure (#350)

* Fixes to build failure on travis (OSX).

* Potential fixes to travis.

* Updated xcode10 images.

Author: dilawar

.travis.yml

@@ -7,7 +7,10 @@ os:
     - linux
     - osx
 
-osx_image: xcode10
+matrix:
+    include:
+        - os : osx
+          osx_image: xcode10
 
 notifications:
   email:

.travis/travis_prepare_osx.sh

@@ -20,9 +20,9 @@
 set -o nounset                              # Treat unset variables as an error
 set +e
 #rvm get head
-#brew update
-#brew outdated cmake || brew install cmake
-brew install gsl
+brew update || echo "failed to update"
+brew outdated cmake || brew install cmake
+brew install gsl || brew upgrade gsl
 brew install hdf5
 brew install python
 brew install numpy