fix for wrong preposition tag

Fix for issue https://github.com/opencog/relex/issues/82


@linas actually i'm not sure whether this fix is correct, please have a closer look. 
The problem was sentence "There were 769 households out of which 38.5% had children under the age of 18 living with them."
creating a unary relation "pronoun-FLAG(household, T)" which is incorrect.

From what I understood the "ref" of the term "which" is modified and pointed to the term "households" through the rules PREP_RELATIVE_REF1, PREP_RELATIVE_REF2 and PREP_RELATIVE_REF3. And finally "pronoun-FLAG(household, T)" is created through rule TAG_PRONOUNS_WH. 

The terms "households" and "of" are connected with a "MJ" connector. But in general purpose of the "MJ\.*" connector is to connect prepositions and other post-nominal modifiers, which is not the case here. So I thought adding "<LAB> != \Mj"  won't do any harm here.