INSTALL.txt is updated to clarify software requirements. 
INSTALL.txt is updated to clarify software requirements. 

git-svn-id: 83a0274dba4329d458142e4079227af35a786d9d@7 2f3f69f2-b874-4715-96d5-35985bcb53a6

allisony/aplpy,mwcraig/aplpy