Switch the Linux Swift tests to Trusty and Swift 4.0.

This uses Travis's new(ish) Ubuntu Trusty image, and changes the
version number of the Swift binaries that we download.

We now install clang-3.6.  This is not the default on Trusty, but
it's required by the Swift 4.0 toolchain.

This also removes the lines to upgrade libicu-dev and sqlite3.  These
are sufficiently up-to-date in the Trusty image.

.travis.yml

@@ -108,6 +108,7 @@ matrix:
         - GROUP=RECURSION
       stage: main-test
     - os: linux
+      dist: trusty
       compiler: clang
       env:
        - TARGET=swift

.travis/before-install-linux-swift.sh

@@ -1,14 +1,12 @@
 set -euo pipefail
 
-# make sure we use trusty repositories (travis by default uses precise)
-curl https://repogen.simplylinux.ch/txt/trusty/sources_c4aa56bd26c0f54f391d8fae3e687ef5f6e97c26.txt | sudo tee /etc/apt/sources.list
-
 # install dependencies
 # some packages below will be update, swift assumes newer versions
 # of, for example, sqlite3 and libicu, without the update some
 # tools will not work
 sudo apt-get update
-sudo apt-get install clang libicu-dev libxml2 sqlite3
+sudo apt-get install clang-3.6 libxml2
+sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-3.6 100
 
 # This would fix a know linker issue mentioned in:
 # https://bugs.swift.org/browse/SR-2299

.travis/run-tests-swift.sh

@@ -4,7 +4,7 @@
 # here since environment variables doesn't pass
 # across scripts
 if [ $TRAVIS_OS_NAME == "linux" ]; then
-  export SWIFT_VERSION=swift-3.1.1
+  export SWIFT_VERSION=swift-4.0
   export SWIFT_HOME=$(pwd)/swift/$SWIFT_VERSION-RELEASE-ubuntu14.04/usr/bin/
   export PATH=$SWIFT_HOME:$PATH