ci: fix http dependency in tests

[ylobankov]

At this moment http dependency in tests is broken since we are trying
to install the http 2.1.0 lua rock. Now this rock is not available
anymore due to tarantool/http#142 and that is the reason for test
failures with the following error:

http not found for Lua 5.1.
Error: No results matching query were found for Lua 5.1.
Checking if available for other Lua versions...
make: *** [Makefile:10: .rocks] Error 1
Checking for Lua 5.2...
Checking for Lua 5.3...
Checking for Lua 5.4...
Error: Process completed with exit code 2.

Now http v2 is available under the http-v2-legacy lua rock and this
patch switches the use of http 2.1.0 to http-v2-legacy 2.1.0.

Manual test run with http-v2-legacy 2.1.0.

[Totktonada]

The http v2 still available, but is not recommended. I suggest to use v1.

$ tarantoolctl rocks search http

http - Search results for Lua 5.1:
==================================


Rockspecs and source rocks:
---------------------------

http
   scm-1 (rockspec) - http://rocks.tarantool.org
   1.1.1-1 (rockspec) - http://rocks.tarantool.org
   1.1.1-1 (src) - http://rocks.tarantool.org
   1.1.0-1 (rockspec) - http://rocks.tarantool.org
   1.0.6-1 (rockspec) - http://rocks.tarantool.org
   1.0.5-1 (rockspec) - http://rocks.tarantool.org
   1.0.4-1 (rockspec) - http://rocks.tarantool.org
   1.0.3-1 (rockspec) - http://rocks.tarantool.org
   1.0.2-1 (rockspec) - http://rocks.tarantool.org
   1.0.1-1 (rockspec) - http://rocks.tarantool.org

http-v2-legacy
   2.1.0-1 (rockspec) - http://rocks.tarantool.org
   2.0.1-1 (rockspec) - http://rocks.tarantool.org
   2.0.0-1 (rockspec) - http://rocks.tarantool.org