modernize .travis.yml

the PPA is no longer recommended, `language: julia` allows testing against more versions of julia

.travis.yml

@@ -1,14 +1,7 @@
-language: cpp
-compiler: 
-    - clang
+language: julia
+julia: 
+    - 0.5
+    - 0.6
+    - nightly
 notifications:
     email: false
-before_install:
-    - sudo add-apt-repository ppa:staticfloat/julia-deps -y
-    - sudo add-apt-repository ppa:staticfloat/julianightlies -y
-    - sudo apt-get update -qq -y
-    - sudo apt-get install libpcre3-dev julia -y
-script:
-    - julia -e 'Pkg.init(); run(`ln -s $(pwd()) $(Pkg.dir())/OSC`); Pkg.resolve()'
-    - julia -e 'using OSC; @assert isdefined(:OSC); @assert typeof(OSC) === Module'
-    - julia test/runtests.jl TEST