Changed `let*` to `let` on solarized-definitions.el

Changed a `let*` to `let` on the `solarized-color-definitions` function.
Without this change, the entire definition of the inner function
`find-color` doesn't compile, with the message "Invalid function" from
emacs.

Tested on:
GNU Emacs 24.2.1 (i386-mingw-nt6.1.7601) of 2012-08-28 on MARVIN