Clean up eigen download code (and bump to 3.3.4)

This changes the travis-ci eigen download code to extract the tar on the
fly (rather than saving to a file first), and extracts into an `eigen`
directory rather than using upstream's `eigen-eigen-xxxxx` directory.

This also bumps the travis-ci eigen release to 3.3.4, in an attempt to
see if it fixed the -Wdeprecated warnings (it did not); the build setup
cleanup seems worth committing anyway.

.travis.yml

@@ -193,9 +193,10 @@ install:
         ${PYPY:+--extra-index-url https://imaginary.ca/trusty-pypi}
         ${PYPY:+--extra-index-url https://imaginary.ca/trusty-pypi}
     echo "done."
     echo "done."
 
 
-    wget -q -O eigen.tar.gz https://bitbucket.org/eigen/eigen/get/3.3.3.tar.gz
-    tar xzf eigen.tar.gz
-    export CMAKE_INCLUDE_PATH="${CMAKE_INCLUDE_PATH:+$CMAKE_INCLUDE_PATH:}$PWD/eigen-eigen-67e894c6cd8f"
+    mkdir eigen
+    curl -fsSL https://bitbucket.org/eigen/eigen/get/3.3.4.tar.bz2 | \
+        tar --extract -j --directory=eigen --strip-components=1
+    export CMAKE_INCLUDE_PATH="${CMAKE_INCLUDE_PATH:+$CMAKE_INCLUDE_PATH:}$PWD/eigen"
   fi
   fi
   set +e
   set +e
 script:
 script: