Issue #21 - Update Firefox to version 55.0

Author: ppodgorsek

Dockerfile

@@ -10,7 +10,7 @@ RUN dnf upgrade -y\
 	&& dnf install -y\
 		chromedriver-59.0.3071.104-1.fc26\
 		chromium-59.0.3071.104-1.fc26\
-		firefox-54.0-2.fc26\
+		firefox-55.0.1-1.fc26\
 		python2-pip-9.0.1-9.fc26\
 		xorg-x11-server-Xvfb-1.19.3-4.fc26\
 		which-2.21-2.fc26\

README.md

@@ -17,7 +17,7 @@ The versions used in the latest version are:
 
 * Robot Framework 3.0.2
 * Robot Framework selenium2library 1.8.0
-* Firefox 54.0
+* Firefox 55.0
 * Chromium 59.0
 
 ## Running the container