requests: Rename http_requests to requests (#619)

The requests integration is named http-requests because at the time it was created there were some problems with pylint. 

other integrations are using opentelemetry.ext.integration without problems, tests are passing without issue, even renamed.

scripts/coverage.sh

@@ -18,7 +18,7 @@ coverage erase
 cov opentelemetry-api
 cov opentelemetry-sdk
 cov ext/opentelemetry-ext-flask
-cov ext/opentelemetry-ext-http-requests
+cov ext/opentelemetry-ext-requests
 cov ext/opentelemetry-ext-jaeger
 cov ext/opentelemetry-ext-opentracing-shim
 cov ext/opentelemetry-ext-wsgi