Ranger build needs gcc... docs/validation

When building ranger on a new Linux system I had a build failure, caused by

[INFO] Executed tasks
[INFO]
[INFO] — native-maven-plugin:1.0-alpha-8:compile (default-compile) @ credValidator —
[INFO] /bin/sh -c cd /home/cloudusr/src/ranger/unixauthnative && gcc -I/home/cloudusr/src/ranger/unixauthnative/src/main/c -o /home/cloudusr/src/ranger/unixauthnative/target/objs/credValidator.o -c /home/cloudusr/src/ranger/unixauthnative/src/main/c/credValidator.c
/bin/sh: 1: gcc: not found

Clearly gcc is needed. Along with java, maven. Listing these prereqs would be useful

Suggest we update the docs at http://ranger.apache.org/quick_start_guide.html
to make this prereq clear.

Ideally update maven to do a check early on