Fix packaging on CentOS 6

[Totktonada]

First, several facts

1. GCC from the base repository on CentOS 6 is GCC 4.4.7.
2. We recently switched from devtoolset-6 to devtoolset-7, because the former seems to be gone.
3. devtoolset-7-gcc on CentOS 6 does not provide gcc (it seems, devtoolset-6-gcc did, but I don't know how to verify).
4. The tarantool-mysql package depends on gcc >= 4.5.

The problem

Now packaging for CentOS 6 fails with the following messages:

Getting requirements for tarantool-mysql-0.0.1-1.el6.src
 --> Already installed : cmake-2.8.12.2-4.el6.x86_64
Error: No Package found for gcc >= 4.5

Proposed solution

Eliminate the dependency on gcc on CentOS 6, just how it is implemented tarantool.