TGM_Plugin_Activation::$menu is always replaced by current ?page=xxx parameter's value due to 

TGM_Plugin_Activation::$menu is always replaced by current ?page=xxx parameter's value due to = (assignment) operator used instead of == or ===. This made install_plugin_page() to be active in all plugin / theme options pages.
Merge pull request #129 from asadkn/patch-1

Only load the Install classes on the Install page.