Set newrelic-install to run in silent mode.

When run without NR_INSTALL_SILENT, the install
acts differently for RHEL7 compared to 6.

On RHEL6, since /bin is not symlinked to /usr/bin,
only 1 instance of php is found, so a prompt 
question doesn’t get asked.

On RHEL7, /bin is symlinked to /usr/bin, 2 instances
of php are found, so a prompt that doesn’t accept
no answer is displayed. This leads to a never ending
question prompt that eats up CPU.

Setting NR_INSTALL_SILENT environment variable
sorts this out.
Merge pull request #30 from andytson/feature/php-agent-silent-install

Set newrelic-install to run in silent mode.