Update less files compilation instructions in INSTALL and UPGRADING

[skip ci]

INSTALL

@@ -60,11 +60,11 @@ INSTALLATION
    IMPORTANT: REMOVE THE INSTALLER DIRECTORY!
    ------------------------------------------
 8. If you use git sources compile css files for the Elastic skin (required
-   lessc >= 1.5.0):
+   lessc >= 2.5.2):
    $ cd skins/elastic
-   $ lessc -x styles/styles.less > styles/styles.css
-   $ lessc -x styles/print.less > styles/print.css
-   $ lessc -x styles/embed.less > styles/embed.css
+   $ lessc --clean-css="--s1 --advanced" styles/styles.less > styles/styles.min.css
+   $ lessc --clean-css="--s1 --advanced" styles/print.less > styles/print.min.css
+   $ lessc --clean-css="--s1 --advanced" styles/embed.less > styles/embed.min.css
 9. Check Known Issues section of this file
 
 

UPGRADING

@@ -59,11 +59,11 @@ it on a unix system, you need to do the following operations by hand:
 4c. If you use git sources or the release package without dependencies
    update javascript dependencies by executing `bin/install-jsdeps.sh` script.
 4d. If you use git sources compile css files for the Elastic skin (required
-   lessc >= 1.5.0):
+   lessc >= 2.5.2):
    $ cd skins/elastic
-   $ lessc -x styles/styles.less > styles/styles.css
-   $ lessc -x styles/print.less > styles/print.css
-   $ lessc -x styles/embed.less > styles/embed.css
+   $ lessc --clean-css="--s1 --advanced" styles/styles.less > styles/styles.min.css
+   $ lessc --clean-css="--s1 --advanced" styles/print.less > styles/print.min.css
+   $ lessc --clean-css="--s1 --advanced" styles/embed.less > styles/embed.min.css
 5. Run `./bin/update.sh` from the commandline OR
    open http://url-to-roundcube/installer/ in a browser and choose "3 Test config".
    To enable the latter one, you have to temporary set 'enable_installer'