Fix oppulance: Explicitly depend on modern flex version

EBRAINS currently has flex@2.6.3, but that version is buggy w.r.t. to "user-defined yy macros" (cf. https://github.com/westes/flex/releases/tag/v2.6.4), which seem to be used by the (particular?) version of binutils used by oppulance.

packages/oppulance/package.py

@@ -14,11 +14,10 @@ class Oppulance(Package):
     depends_on('gettext')
     depends_on('zlib')
     depends_on('bison')
-    depends_on('flex')
+    depends_on('flex@2.6.4:')
     depends_on('m4')
     depends_on('texinfo')
     depends_on('wget')
-    conflicts('flex', when='@2.6.3', msg='Binutils 2.25 for Nux doesn\'t build with flex 2.6.3.')
 
     releases = [
         {