Fix make test on Debian / Ubuntu

Debian and Debian-based distros has dash as /bin/sh implementation
(whereas it seems that all other distros just use bash), so we need to
use Bourne Shell compliant `.` command to source a file.

Thanks Alexey Khatskevich (@Khatskevich) to pointing it out!

Author: Totktonada

Makefile

@@ -8,7 +8,7 @@ lint:
 .PHONY: test
 test: lint
 	virtualenv -p python2.7 ./.env-2.7
-	source ./.env-2.7/bin/activate && \
+	. ./.env-2.7/bin/activate && \
 		pip install -r ./test-run/requirements.txt && \
 		pip install tarantool && \
 		cd test && ./test-run.py