Updates for LOOT's Travis CI integration

* Build and use Boost 1.61.0
* Cache Node, Bower and Boost dependencies
* Always link to static Boost libraries
* Remove Travis's Boost ABI incompatibility hack

scripts/travis/install_boost.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+
+BOOST_BASENAME=boost_1_61_0
+BOOST_LIBRARIES=(libboost_atomic.a libboost_chrono.a libboost_date_time.a libboost_filesystem.a libboost_iostreams.a libboost_locale.a libboost_log.a libboost_log_setup.a libboost_regex.a libboost_system.a libboost_thread.a)
+
+function isLibraryMissing {
+  for LIBRARY in $BOOST_LIBRARIES; do
+    if [[ ! -e ~/$BOOST_BASENAME/stage/lib/$LIBRARY ]]; then
+      return 0
+    fi
+  done
+
+  return 1
+}
+
+if isLibraryMissing; then
+  cd ~
+
+  wget http://downloads.sourceforge.net/project/boost/boost/1.61.0/${BOOST_BASENAME}.tar.bz2
+  tar xf ${BOOST_BASENAME}.tar.bz2
+
+  cd $BOOST_BASENAME
+  ./bootstrap.sh
+  ./b2 toolset=gcc-5 link=static variant=release address-model=64 cxxflags="-std=c++14 -fPIC" boost.locale.icu=off --with-log --with-date_time --with-thread --with-filesystem --with-locale --with-regex --with-system --with-iostreams
+fi