Switch CircleCI to only do release builds. It doesn't suffer from

timeouts and is generally faster VMs. But only handles 4 builds at a
time.

circle.yml

@@ -6,7 +6,7 @@
 
 machine:
   environment:
-    SCRIPT_N: ci_boost_status ci_boost_library_check ci_boost_release ci_boost_release
+    SCRIPT_N: ci_boost_release ci_boost_release
     EOL_N: NA NA LF CRLF
   pre:
     - |