HOUSTON — Three days after they were swept by Colorado, the Pittsburgh Pirates feel much better after doing the same to Houston. Jason Bay hit a two-run homer and Pittsburgh earned its first road sweep this season with an 8-7 win over the Astros on Wednesday. Doug Mientkiewicz drove in two runs, starter Ian Snell had an RBI single and Franquelis Osoria (4-3) pitched two innings of scoreless relief as Pittsburgh finished off its first sweep since taking three from San Francisco at PNC Park from May 6-8. ‘‘As bad as we played in Colorado, to turn around and do this three days in a row was important,’’ said Mientkiewicz, who had four hits. ‘‘We needed these three bad.’’ Damaso Marte shut out the Astros in the ninth for his fifth save in seven chances. The Pirates have won three straight road games for the first time this season and have their first three-game victory streak since taking six straight from May 6-12.