GenericValue: reduce growth factor for array/object reallocations

As mentioned by @kosta-github in http://git.io/0gkYSg, the currently
used growth factor of 2 is suboptimal for memory performance.  An
extensive discussion can be found at [1].

This patch reduces the array/object capacity growth factor to 1.5, as
many C++ implementations have chosen to use.  In order to avoid
floating-point arithmetics for computing the new capacity, I did not
add any customization parameter for the factor and used a shift+add
instead.

[1] https://github.com/facebook/folly/blob/master/folly/docs/FBVector.md
GenericValue: round up during capacity growth

Suggested-by: @miloyip
Merge pull request #130 from pah/fixes/capacity-growth

GenericValue: reduce growth factor for array/object reallocations