Modify Travis configuration to install GSL library

Author: jiahao

.travis.yml

@@ -1,4 +1,8 @@
 language: julia
+addons:
+    apt:
+        packages:
+            - libgsl0-dev
 os:
     - linux
 julia:
@@ -11,5 +15,5 @@ script:
     - if [[ -a .git/shallow ]]; then git fetch --unshallow; fi
     - julia -e 'Pkg.clone(pwd()); Pkg.build("RandomMatrices"); Pkg.test("RandomMatrices"; coverage=true)'
 after_success:
-    - julia -e 'Pkg.add("Coverage"); cd(Pkg.dir("RandomMatrices")); using Coverage; Coveralls.submit(Coveralls.process_folder()); Codecov.submit(process_folder())'
+    - julia -e 'Pkg.add("Coverage"); cd(Pkg.dir("RandomMatrices")); using Coverage; Coveralls.submit(process_folder()); Codecov.submit(process_folder())'