.travis.yml: Replace manual zlib installation with "dist: trusty"

Travis now offers Ubuntu Trusty (14.04 LTS) VMs as test runners, which
is gradually becoming the default. We can opt in to using Trusty now
so that we no longer need to manually update zlib to a newer version.

.travis.yml

@@ -1,17 +1,12 @@
 language: c
+
+dist: trusty
+sudo: required
+
 before_install:
   - sudo apt-get update -qq
   - sudo apt-get install dtach libxapian-dev libgmime-2.6-dev libtalloc-dev python-sphinx gdb gpgsm
 
-  # Notmuch requires zlib 1.2.5.2, unfortunately travis runs on Ubuntu 12.04LTS which
-  # ships with zlib 1.2.3.3. We need to update to zlib 1.2.5.2 to be able to build.
-  # TODO: Watch https://github.com/travis-ci/travis-ci/issues/2046 and remove
-  #       this hack once travis-ci switches to Ubuntu 14.04
-  - wget 'https://github.com/notmuch/travis-files/raw/master/zlib1g-dev_1.2.8.dfsg-1ubuntu1_amd64.deb'
-  - wget 'https://github.com/notmuch/travis-files/raw/master/zlib1g_1.2.8.dfsg-1ubuntu1_amd64.deb'
-  - sudo dpkg -i zlib1g-dev_1.2.8.dfsg-1ubuntu1_amd64.deb zlib1g_1.2.8.dfsg-1ubuntu1_amd64.deb
-  - sudo apt-get install -f
-
 script:
   - ./configure
   - make download-test-databases