fix version number in userAgent

v7.8.0 (#213)

bump version to v7.7.0 (#209)

bump version to v7.6.0 (#207)

Bump version to 7.5.0 (#202)

Merge pull request #198 from qiniu/upgrades/7.4.0

bump version to v7.4.0

Update CHANGELOG.md

Update conf.c

Update CHANGELOG.md

Update conf.c