fixing readme and configure script links to double-conv

Summary:
Fixing following the issues raised by nsuke on github
following his diff (https://github.com/facebook/folly/pull/112)

Test Plan: visual inspection, tested build on arch linux

Reviewed By: davejwatson@fb.com

Subscribers: folly-diffs@

FB internal diff: D1807378

Tasks: 5909189

Signature: t1:1807378:1422403093:ca024f396a27ecd9b304c57643f0107764a38ca1

folly/configure.ac

@@ -77,7 +77,7 @@ AC_CHECK_HEADERS([fcntl.h features.h inttypes.h limits.h stdint.h stdlib.h strin
 
 AC_CHECK_HEADER(double-conversion/double-conversion.h, [], [AC_MSG_ERROR(
                 [Couldn't find double-conversion.h, please download from \
-                http://code.google.com/p/double-conversion/])], [])
+                 https://github.com/floitsch/double-conversion/])], [])
 AC_CHECK_LIB([double-conversion],[ceil],[],[AC_MSG_ERROR(
              [Please install double-conversion library])])