I've spent half my day solving this. I think it would at least be helpful to others who get stuck at plugin installation related version requirement problems with Redmine.
If you thoroughly inspect the plugin.rb file from the error message, you'll see that the requirement statement is in the init.rb file. Its path can be found in plugin.rb's initializer section which is on line 123. It was a little bit hard for me to understand that it pointed to the plugin's own root path. There I found the main init.rb file, and in it was again an initializer section on line 173 which pointed to my final destination which was defined a few lines before it. The lines before the initializer section do define some paths which in the end brought me to "…/plugins/easyproject/easy_plugins/easy_extensions/init.rb".
This file's last line reads as "requires_redmine :version_or_higher => '2.5.1'" which is where you put your magic into.
After I edited it to '2.5.0' the bundle rake install command did respond correctly by migrating the plugin.
Now my Redmine has the plugin and works flawlessly but the only part that I'm not sure is if this will cause version incompatibility problems in the future.
I'll report back if I encounter a problem. I already asked easyredmine support about it and waiting for their reply. There are very few changes between 2.5.0 and 2.5.1 which comforts me a little bit.