Modify Travis script so Eigen is fetched using ExternalProject.
Modify Travis script so Eigen is fetched using ExternalProject.

kartikkumar/sml,ennehekma/sml,astropnp/sml,Stardust2013/sml,astropnp/sml,ennehekma/sml,openastro/sml,Stardust2013/sml