travis: drop support for Ubuntu Precise

Ubuntu Precise is end of life since April 2017. At that point in time,
Precise was still the main distro on which Travis CI built upon, with
the Trusty-based images still being in a beta state. But since June
21st, Trusty has officially moved out of beta and is now the default
image for all new builds. Right now, we build on both old and new images
to assure we support both.

Unfortunately, this leaves us with the highest minimum version for CMake
being 2.8.7, as Precise has no greater version in its repositories. And
because of this limitation, we cannot actually use object libraries in
our build instructions. But considering Precise is end of life and
Trusty is now the new default for Travis, we can and should drop support
for this old and unmaintained distribution. And so we do.

.travis.yml

@@ -31,10 +31,6 @@ matrix:
      compiler: gcc
  include:
    - compiler: gcc
-     env: PRECISE=1
-     os: linux
-     dist: precise
-   - compiler: gcc
      env: COVERITY=1
      os: linux
      dist: trusty

script/install-deps-linux.sh

@@ -2,11 +2,4 @@
 
 set -x
 
-if [ -z "$PRECISE" ]; then
-    echo "deb http://libgit2deps.edwardthomson.com trusty libgit2deps" | sudo tee -a /etc/apt/sources.list
-    sudo apt-key adv --keyserver pgp.mit.edu --recv 99131CD5
-    sudo apt-get update -qq
-    sudo apt-get install -y curl libcurl3 libcurl3-gnutls libcurl4-gnutls-dev
-fi
-
 sudo apt-get install -y cmake libssh2-1-dev openssh-client openssh-server valgrind