Update README not to install from rubygems.

Hi, could you consider to merge this change?

When I installed and used gridinit-jmeter in accordance with README, it resulted to create corrupted jmx file like the following

```xml
  <HTTPSamplerProxy guiclass="HttpTestSampleGui" testclass="HTTPSamplerProxy" testname="{:name=&gt;"/>
```

To solve it at this time, we need to install the newest version supporting keyword arguments.