Specify all required packages in `.travis.yml`

- most seem to be included in current Travis bundles but Trusty support is in Beta

.travis.yml

@@ -4,7 +4,12 @@ dist: trusty
 addons:
   apt:
     packages:
-      - libunwind8
+    - gettext
+    - libcurl4-openssl-dev
+    - libicu-dev
+    - libssl-dev
+    - libunwind8
+    - zlib1g
 env:
   - KOREBUILD_DNU_RESTORE_CORECLR=true KOREBUILD_TEST_DNXCORE=true
 install: