Summary:
The code in `FindDoubleConversion.cmake` called
`FIND_PACKAGE_HANDLE_STANDARD_ARGS()` with the wrong arguments.  The first
argument must match the package name: since this file is called
`FindDoubleConversion` the first argument must be `DoubleConversion`.

Previously this code was passing in `DOUBLE_CONVERSION`, causing it to set
`DOUBLE_CONVERSION_FOUND` instead of `DoubleConversion_FOUND`.  Since this
module did not set `DoubleConversion_FOUND` the `find_package()` call assumed
it always succeeded, even when the double-conversion library was not found.

Also remove the extra status message.  `FIND_PACKAGE_HANDLE_STANDARD_ARGS()`
already prints out a status message as long as `find_package()` was called
without the `QUIET` argument.

Reviewed By: yfeldblum

Differential Revision: D23588980

fbshipit-source-id: 8c088930b1b01c027403c3e425f1924f6149f72a