that definitely seems to be the problem, as when I go into terminal (Console on windows) and input "java -version" it responds with "Java(TM) SE Runtime Environment (build 1.8.0_40-b27)", but the Minecraft launcher log still says "System.getProperty('java.version') == '1.6.0_65' "
On the mc launcher, go to "edit profile". If you look, the should be a box that says something like Java version. If you've downloaded the 8 JDK, you just need to paste the file path. I think I have a sub thread a few pages ago when I had the same issue. I'll see if I can relocate it.