Merge pull request #1000 from tmciver/readme-fix

Had to also install libgmp-dev on Ubuntu 18.04.

Author: alanz

README.md

@@ -111,7 +111,7 @@ On Linux you will need install a couple of extra libraries (for Unicode ([ICU](h
 **Debian/Ubuntu**: 
 
 ```bash
-sudo apt install libicu-dev libtinfo-dev
+sudo apt install libicu-dev libtinfo-dev libgmp-dev
 ```
 **Fedora**: