Use Ruby 2.1.7 for new installs from source

doc/install/installation.md

@@ -115,8 +115,9 @@ Remove the old Ruby 1.8 if present
 Download Ruby and compile it:
 
     mkdir /tmp/ruby && cd /tmp/ruby
-    curl -L --progress http://cache.ruby-lang.org/pub/ruby/2.1/ruby-2.1.6.tar.gz | tar xz
-    cd ruby-2.1.6
+    curl -O --progress https://cache.ruby-lang.org/pub/ruby/2.1/ruby-2.1.7.tar.gz
+    echo 'e2e195a4a58133e3ad33b955c829bb536fa3c075  ruby-2.1.7.tar.gz' | shasum -c - && tar xzf ruby-2.1.7.tar.gz
+    cd ruby-2.1.7
     ./configure --disable-install-rdoc
     make
     sudo make install