Add gmp-5.1.3 dependency to Travis

The double factorial function requires GMP >=5.1.0; Travis on
Ubuntu-precise only offers 5.0.2.

Ref:
JuliaLang/METADATA.jl@169d630

Author: jiahao

.travis.yml

@@ -10,7 +10,7 @@ before_install:
     - sudo add-apt-repository ppa:staticfloat/julia-deps -y
     - sudo add-apt-repository ppa:staticfloat/${JULIAVERSION} -y
     - sudo apt-get update -qq -y
-    - sudo apt-get install libpcre3-dev julia -y
+    - sudo apt-get install libgmp10 libpcre3-dev julia -y
     - git config --global user.name "Travis User"
     - git config --global user.email "[email protected]"
     - if [[ -a .git/shallow ]]; then git fetch --unshallow; fi