Update README.md to include ansible version

Ansible version needs to be >= 1.7, otherwise mesos_install_mode="slave" does not get set correctly, and slaves get set up as master nodes.

:-1: ANSIBLE 1.6:
````
TASK: [ansible-mesos | debug msg="Install Mode is slave"] *********************
ok: [ec2-54-87-91-235.compute-1.amazonaws.com] => {
    "item": "",
    "msg": "Install Mode is master"
}
ok: [ec2-54-235-237-69.compute-1.amazonaws.com] => {
    "item": "",
    "msg": "Install Mode is master"
}
ok: [ec2-54-234-24-109.compute-1.amazonaws.com] => {
    "item": "",
    "msg": "Install Mode is master"
}
````

:+1: ANSIBLE 1.7:
````
TASK: [ansible-mesos | debug msg="Install Mode is slave"] *********************
ok: [ec2-54-87-91-235.compute-1.amazonaws.com] => {
    "msg": "Install Mode is slave"
}
ok: [ec2-54-235-237-69.compute-1.amazonaws.com] => {
    "msg": "Install Mode is slave"
}
ok: [ec2-54-234-24-109.compute-1.amazonaws.com] => {
    "msg": "Install Mode is slave"
}
````