Convert some /usr/bin/which calls to use the 'which' method

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/couchdb-lucene.rb

@@ -37,7 +37,7 @@ class CouchdbLucene < Formula
 os_process_timeout=60000 ; increase the timeout from 5 seconds.
 
 [external]
-fti=#{`which python`.chomp} #{prefix}/tools/couchdb-external-hook.py
+fti=#{which 'python'} #{prefix}/tools/couchdb-external-hook.py
 
 [httpd_db_handlers]
 _fti = {couch_httpd_external, handle_external_req, <<"fti">>}

Formula/cvs2svn.rb

@@ -5,7 +5,7 @@ class PythonWithGdbm < Requirement
     The Python being used does not include gdbm support,
     but it is required to build this formula:
 
-      #{`which python`.chomp}
+      #{which 'python'}
 
     Homebrew's Python includes gdbm support.
     EOS

Formula/rrdtool.rb

@@ -26,8 +26,8 @@ class Rrdtool < Formula
     ENV.libxml2
     ENV.x11
 
-    which_perl = `/usr/bin/which perl`.chomp
-    which_ruby = `/usr/bin/which ruby`.chomp
+    which_perl = which 'perl'
+    which_ruby = which 'ruby'
 
     opoo "Using system Ruby. RRD module will be installed to /Library/Ruby/..." if which_ruby == "/usr/bin/ruby"
     opoo "Using system Perl. RRD module will be installed to /Library/Perl/..." if which_perl == "/usr/bin/perl"