Add libeigen3-dev installation package to .travis.yml rules
Add libeigen3-dev installation package to .travis.yml rules

YARP complains about missing Eigen headers. From v2.3.68 release notes:

	libYARP_math can be compiled with GSL or Eigen (via CMake flag
	CREATE_LIB_MATH_USING_GSL). Important: the use of GSL and this flag
	will be removed in the next release of YARP.

	* When compiled with GSL libYARP_math uses libYARP_gsl internally. A
	small performance hit could affect methods that use GSL.
	* When compiled without GSL libYARP_math uses Eigen.

* https://github.com/robotology/yarp/releases/tag/v2.3.68
* https://travis-ci.org/asrob-uc3m/robotDevastation/jobs/91351705
* http://packages.ubuntu.com/precise/libeigen3-dev
asrob-uc3m/robotDevastation,asrob-uc3m/robotDevastation