Move Travis to supported Linux distribution

- use Ubuntu 14.04 (Trusty)
  - Travis support for Trusty is in Beta and currently requires `sudo`
- run `dnu restore` with DNX Core since aspnet/External#49 is not fixed in Mono versions we can use
- add required dependencies for DNX Core to `.travis.yml`
- addresses part of aspnet/Universe#290

.travis.yml

@@ -1,6 +1,18 @@
 language: csharp
-sudo: false
+sudo: required
+dist: trusty
+addons:
+  apt:
+    packages:
+    - gettext
+    - libcurl4-openssl-dev
+    - libicu-dev
+    - libssl-dev
+    - libunwind8
+    - zlib1g
+env:
+  - KOREBUILD_DNU_RESTORE_CORECLR=true
 mono:
   - 4.0.5
 script:
-  - ./build.sh --quiet verify
+  - ./build.sh --quiet verify