Update install.rst
Update install.rst

- Added hint regarding pip install on low memory machines.
- Added hint to python 3 version of pandas from distributon repos.

pandas-dev/pandas,harisbal/pandas,MJuddBooth/pandas,toobaz/pandas,harisbal/pandas,cython-testbed/pandas,dsm054/pandas,gfyoung/pandas,jreback/pandas,nmartensen/pandas,TomAugspurger/pandas,jorisvandenbossche/pandas,rs2/pandas,nmartensen/pandas,cython-testbed/pandas,pratapvardhan/pandas,linebp/pandas,kdebrab/pandas,dsm054/pandas,louispotok/pandas,rs2/pandas,gfyoung/pandas,DGrady/pandas,zfrenchee/pandas,Winand/pandas,gfyoung/pandas,linebp/pandas,kdebrab/pandas,zfrenchee/pandas,DGrady/pandas,winklerand/pandas,jmmease/pandas,linebp/pandas,zfrenchee/pandas,GuessWhoSamFoo/pandas,GuessWhoSamFoo/pandas,MJuddBooth/pandas,dsm054/pandas,kdebrab/pandas,TomAugspurger/pandas,jorisvandenbossche/pandas,datapythonista/pandas,cython-testbed/pandas,louispotok/pandas,winklerand/pandas,linebp/pandas,amolkahat/pandas,zfrenchee/pandas,harisbal/pandas,winklerand/pandas,harisbal/pandas,Winand/pandas,jmmease/pandas,pratapvardhan/pandas,jreback/pandas,pandas-dev/pandas,Winand/pandas,rs2/pandas,TomAugspurger/pandas,gfyoung/pandas,toobaz/pandas,MJuddBooth/pandas,louispotok/pandas,kdebrab/pandas,cbertinato/pandas,pandas-dev/pandas,datapythonista/pandas,linebp/pandas,datapythonista/pandas,jreback/pandas,winklerand/pandas,GuessWhoSamFoo/pandas,harisbal/pandas,pratapvardhan/pandas,toobaz/pandas,cbertinato/pandas,MJuddBooth/pandas,cbertinato/pandas,amolkahat/pandas,linebp/pandas,amolkahat/pandas,toobaz/pandas,winklerand/pandas,TomAugspurger/pandas,winklerand/pandas,cython-testbed/pandas,MJuddBooth/pandas,Winand/pandas,nmartensen/pandas,nmartensen/pandas,DGrady/pandas,DGrady/pandas,pandas-dev/pandas,cython-testbed/pandas,nmartensen/pandas,DGrady/pandas,cbertinato/pandas,amolkahat/pandas,GuessWhoSamFoo/pandas,amolkahat/pandas,datapythonista/pandas,jmmease/pandas,jreback/pandas,jmmease/pandas,gfyoung/pandas,jmmease/pandas,jmmease/pandas,louispotok/pandas,jreback/pandas,DGrady/pandas,cbertinato/pandas,dsm054/pandas,Winand/pandas,zfrenchee/pandas,kdebrab/pandas,louispotok/pandas,pratapvardhan/pandas,dsm054/pandas,rs2/pandas,jorisvandenbossche/pandas,toobaz/pandas,Winand/pandas,GuessWhoSamFoo/pandas,pratapvardhan/pandas,nmartensen/pandas,jorisvandenbossche/pandas