Update guile version dependency info in docs from "1.8.2 or newer" to "1.8.2"

[Paul Morris]

In the docs, update dependency info from guile “(1.8.2 or newer)” to just 
“(1.8.2)” until guile 2.0 support is stable.  As reported on the user list here:

http://lilypond.1069038.n5.nabble.com/Kubuntu-GCC-compile-with-guile-2-td169498.html
 
<http://lilypond.1069038.n5.nabble.com/Kubuntu-GCC-compile-with-guile-2-td169498.html>


I did a cursory web search and found these two instances. There may be others.

http://lilypond.org/doc/v2.18/Documentation/topdocs/INSTALL#requirements-for-running-lilypond
 
<http://lilypond.org/doc/v2.18/Documentation/topdocs/INSTALL#requirements-for-running-lilypond>

http://lilypond.org/doc/v2.19/Documentation/contributor-big-page#requirements-for-running-lilypond
 
<http://lilypond.org/doc/v2.19/Documentation/contributor-big-page#requirements-for-running-lilypond>